The present invention relates to a digital pulse processor and, more particularly, relates to a processor that uses multiple parameters and criteria for rejecting piled or contaminated pulses.
In electronic sensing applications where information occurs randomly in time, such as in nuclear processes, there is a finite and calculable probability that two or more discrete pulse events will overlap to form a contaminated or xe2x80x9cpiled-upxe2x80x9d pulse event. Similarly, in sensing applications involving two or more constant frequency but asynchronous sources whose inputs are summed into one, there is also a calculable probability of a piled-up pulse event.
Many such applications require integration of a pulse over a time interval to determine its energy content or to analyze its shape for Doppler or other determinations. If the pulse is contaminated or piled-up, a significant decrease in measurement quality will result. Consider, for example, an example from the field of nuclear spectroscopy. A first X- or Gamma-ray of energy E1 enters a sensor at time T1, and a second ray of energy E2 enters the sensor at time T2, where T2xe2x88x92T1 is less than the integration time IT. The energy integral is a composite of rays E1 and E2, and is accumulated in the spectrum being collected as a pseudo ray of energy E3, where E1 less than E3 less than =E1+E2. This contamination of the spectrum decreases the accuracy of the resultant spectrographic analysis.
FIGS. 1a-1c graphically depict the phenomenon of pulse pileup. In FIGS. 1a-1c, the horizontal axis represents time (ns) and the vertical axis represents the digitized pulse amplitude. Eight digitized pulses 1-8 are depicted, with the pulses being numbered in time sequence order. FIG. 1a plots pulses 1, 3, 5 and 8 and FIG. 1b plot pulses 2, 4, 6 and 7. In the first plot, pulses 1, 3, 5 and 8 are single events contained within integration zones depicted by the vertical dashed lines. Similarly, in the second plot, pulses 2, 4, 6 and 7 are single events contained within integration zones depicted by the vertical dashed lines. The energy of the X- or Gamma-ray that pulses 1-8 might represent is determined by computing the sum of n-points of digitized pulse data within an integration zone, dividing this sum by a fixed number, and truncating the result to an integer. The resultant integer is an energy bin or channel into which the pulse event is placed to become a part of the spectra.
FIG. 1c plots the combination of pulses 1-8. The overlap of the pulses within the integration zones results in four pulses: pulse 1,2 is the combination of pulses 1 and 2; pulse 3,4 is the combination of pulses 3 and 4; pulse 5,6 is the combination of pulses 5 and 6; and pulse 7,8 is the combination of pulses 7 and 8. In a sensor that cannot determine that these pulses are not from single rays, i.e. a sensor without pileup rejection, pulses 1,2; 3,4; 5,6; and 7,8 would be processed as single pulses and be recorded in the measured spectra. Since these piled-up pulses are not analytically meaningful, however, the spectra of singularly measured rays are contaminated and spectral analysis errors result.
In random processes, such as those encountered in nuclear applications, POISSON Statistics are used to calculate the probability of pulse pileup or contamination during an integration period. POISSON Statistics are explained in detail in Glenn F. Knoll, xe2x80x9cRadiation Detection and Measurementxe2x80x9d, 2nd Edition, John Wiley and Sons, New York, 1989, pages 96-97 (hereinafter xe2x80x9cKnollxe2x80x9d). Over an integration period t with an average count rate r, the probability that a first event will be contaminated by a second event is given by equation 1:
P(r,t)=exe2x88x92rt. 
Equation 1 is derived by integrating Equation 3-60 on page 97 of Knoll over the interval 0 to t.
Table 1, which follows this detailed description, outlines the pulse pileup probability for various integration periods and count rates, calculated using Equation 1. For high count rates, pulse pileup occurs a relatively large percentage of the time. For example, referring to Table 1, if the count rate is 400,000 events/second, and the integration period is 400 ns, P(400000,400 ns)=0.147856, meaning that there will be pulse pileup 14.78% of the time. For low count rates, conversely, the probability of pulse pileup is much lower. For a count rate of 10,000 events/second and an integration period of 400 ns, P(10000,400 ns)=0.003992, or only 0.4% of the time. Similarly, as the integration period is increased, the pulse pileup probability also increases.
Before pulse digitization and microprocessor integration techniques were available, analog methods were used. Conventional analog methods employed an integrating operational amplifier. After a pulse passes through the amplifier, the voltage on the output of the amplifier is proportional to the area under the pulse. A multi-channel analyzer read this voltage and tallied the events in electronic memory bins or channels. The number of the bin or channel was proportional to the size of integrated voltage and the energy or size of the pulse event. The result was a spectrum of energy or size. Between pulses, the integrating capacitor on the amplifier was shorted to ground to discharge the integral voltage.
Analog techniques such as this also have difficulty in detecting pulses that have been contaminated with pileup from other randomly occurring events, particularly small pulses overlaid on large pulses, and vice-versa. One type of analog pileup rejection utilizes a constant fraction timing approach on both the leading and trailing edges of the analog pulse to determine key aspects of the pulse""s shape. This method has been commercialized and is discussed in xe2x80x9cModular Pulse-Processing Electronics and Semiconductor Radiation Detectorsxe2x80x9d, EGandG ORTEC, 100 Midland Road, Oak Ridge, Tenn. 37831, 1997/98 catalog, pages 2.234-2.236 and 2.242-2.243. A second type of analog pileup rejection is described by Marshall, U.S. Pat. No. 4,152,596. This patent describes a system utilizing slow and fast amplifiers and a pulse width determining means, wherein for a pulse to be classified as accepted as a single event, it was required that (1) the amplitudes of the outputs of both amplifiers be consistent with a single event producing both outputs, and (2) the width of the output pulse from the fast amplifier must also be consistent with that of a single event.
In view of the above, it can be seen that a means for reliably filtering or rejecting contaminated or piled-up pulses is necessary for accurate and quality pulse processing and analysis.
An extremely sensitive method and apparatus for rejection of pulses contaminated by pulse pileup or other interference is provided. In spectroscopy, the present invention greatly improves the purity and quality of spectra. The invention is especially useful in high count rate applications, where the probability of random pulse overlap during the energy integration period is significant. The present invention can be implemented off-line or in real-time without loss of throughput. A plurality, preferably several, parameters that characterize the shape of ideal or non-piled pulses are chosen. The parameters are chosen to effectively discriminate piled pulses from non-piled pulses. Typically, these parameters are checked against measured values for non-piled pulses that are stored in a lookup table or library. Statistical multipliers of the standard deviations of each measured parameter are typically use to control the rejection sensitivity. The method utilizes digitized pulses or portions of pulses.
Thus, in one aspect, the present invention provides a method for processing a pulse. The method involves comparing at least one, but preferably a plurality of parameters for a pulse, with those parameters for a non-piled pulse. The comparison for a parameter can be performed in various ways, for example, by comparison of a parameter determined or measured for a pulse with a value of that parameter in a lookup table or library, or by calculating the value of the parameter for a non-piled pulse. The comparison and/or the determination of a parameter can include determining a relationship between other parameters of the pulse. The pulse processing can be performed, for example, in a dedicated processor, a general purpose computer, or a combination.
The plurality of parameters can be two parameters, but is preferably 3 or 4 parameters, but can also be 5 or 6 or more parameters.
As recognized by those skilled in the art, the logic and circuitry for implementing the present method can be designed in many different ways all within the scope of the present invention.
Thus, in a preferred embodiment, the invention provides a method for discriminating an uncontaminated single event pulse from a piled pulse by
(a) calculating a plurality of pulse shape description parameters for a plurality of regions of a digitized pulse;
(b) comparing the calculated parameters to parameter criteria for the same regions of an unpiled pulse;
(b) accepting the pulse as an uncontaminated single event pulse if the calculated parameters satisfy the parameter criteria and rejecting the pulse if the calculated parameters do not satisfy the parameter criteria; and
(c) storing the pulse in a first spectral memory bank if the pulse is accepted. The pulse can be stored in a second, third, or (nth+1) spectral memory bank if the calculated parameter does not meet the criteria, or immediately rejected.
Also in preferred embodiments, the method comprises the following steps:
(a) sensing the pulse;
(b) digitizing the pulse;
(c) calculating a parameter from a region of the pulse;
(d) comparing the calculated parameter to a criteria from the same region of at least one known, non-piled pulse;
(e) accepting and storing the pulse in a first spectral memory bank if the calculated parameter meets the criteria; and
(f) rejecting and storing the pulse in a second, third, or (nth+1) spectral memory bank if the calculated parameter does not meet the criteria.
Clearly, the above process can be varied. For example, preferably a plurality of parameters are calculated and compared. The parameter calculations can all be performed and then all comparisons performed, or each parameter can be calculated and compared in turn, or in combinations. Also, calculation and comparison of one or more parameters can be optional. For example, the parameter(s) can be utilized only in cases where previous comparison of a different parameter(s) provides an ambiguous result on whether a pulse should be accepted or rejected. This would occur, for example, where the acceptance/rejection criteria utilized two different thresholds, leaving a range where a pulse is neither accepted nor rejected. Also, in the exemplary process above, step (f) is optional. Instead of storing a rejected pulse in a secondary spectral memory bank, the pulse can be discarded immediately. Also, the pulse region for calculating a parameter can be selected to be of various widths. Further, the range of acceptable values for a parameter can be adjusted as desired, e.g., based on empirical determination of variability with un-piled pulses for that parameter for a particular analysis.
In another aspect of the present invention, a pulse processor is provided. It comprises a pulse sensor for sensing pulses and an analog-to-digital converter that digitizes the sensed pulses into discrete ADC values over a plurality of time slices. A first storage medium stores the digitized pulses. A classification processor comprises means for calculating at least one parameter from the time slices in at least one region of the pulses and means for comparing the calculated parameter to criteria from the same region of at least one known, non-piled pulse. The processor further comprises a first spectral memory bank for storing accepted pulses whose calculated parameters meet the criteria; and a second, third, or (nth+1) spectral memory bank for storing rejected pulses whose calculated parameters do not meet the criteria. The pulse processor can be incorporated in an analyzer system, e.g., a neutron activation analyzer system, such as a PGNAA analyzer. More generally, the pulse processor (and the method described herein) can also be incorporated in any analyzing system that requires the processing of single events that suffer multiple event pileup or overlap. These include, without limitation, (1) nuclear gauges and devices utilizing X-rays or Gamma rays, measuring density, thickness, weight, composition and/or spectra, wherein the sensed radiation occurs randomly in time and inherently generates pulse pileup, and (2) sonar or radar applications wherein normal single event reflections can suffer overlap and pileup due to interfering reflections.
The method and processor indicated above and described herein can be utilized in many different applications and analysis systems, Thus, in another aspect, the invention provides an analyzer, for example, a Prompt Gamma Neutron Activation Analysis (PGNAA) analyzer which includes a pulse processor and/or utilizes the pulse processing method as indicated in the preceding aspects and described herein. The analyzer may be of many different types and configurations, e.g., including systems as described in patents cited herein, as well as in other nuclear spectroscopy systems, among other applications.
In related aspects, the invention provides other analyzers or devices or methods which incorporate the method or processor of this invention in which a single event signal or pulse can be piled or contaminated by other single event pulses, and/or in which a detected reflected electromagnetic signal can be contaminated by secondary reflections. Examples of the latter include sonar and radar applications. In connection with sonar and radar, as well as other applications, the signal is typically a stream or signal of several or many wave periods (the signal thus comprises a plurality of waves). The present analysis method can be applied in such contexts by analyzing the signal wave by wave by comparison of wave portion parameters between a detected signal and an uncontaminated signal.
Objects and advantages of the present invention include any of the foregoing, singly or in combination. Further objects and advantages will be apparent to those of ordinary skill in the art, or will be set forth in the following disclosure.